SolarSoft

HESSI Analysis GuideHigh Energy Solar Spe tros opi Imager

Ver 0.?? Ed: R.D. Bentley - Date Below

This Copy Produ ed: May 31, 2001

Prepared at
Mullard Spa e S ien e Laboratory
University College London

CONTENTS

i

Contents
1 Introdu tion

1

2 HESSI User Guide

3

1.1
1.2
1.3
1.4
1.5

The HESSI Mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HESSI Instrument Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Referen es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conta ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A knowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.0.1 Environment Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1 Sele ting and Reading Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 The HESSI Observing Summary . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Using the HESSI Flare List . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Using a Map Obje t or an SSW Index Re ord . . . . . . . . . . . . . . . .
2.2 Time plots of HESSI Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Retrieving parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Image Re onstru tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Re onstru tion Te hniques . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Verifying the Image quality . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3 Keywords related to Imaging . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Spe tros opy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Keywords related to Spe tros opy . . . . . . . . . . . . . . . . . . . . . .
2.5 More Complex Data produ ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1 Imaging Spe tros opy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.2 Image Cube Light urves . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6 Comparing HESSI with Other Data . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1 Comparing with Images . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.2 Comparing Time series . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7 HESSI Synopti Data Ar hive . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8 HESSI Graphi User Interfa e (GUI) . . . . . . . . . . . . . . . . . . . . . . . . .
2.9 Simulating HESSI Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.10 Software Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 HESSI Instrument Guide

3.1 The HESSI Observatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Instrument Des ription . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2 Spa e raft Pointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 HESSI Instrument Subsystems . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Imaging System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Germanium Dete tors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.4 Solar Aspe t Sensor (SAS) . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.5 Roll Angle Sensor (RAS) . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.6 Data Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 HESSI Instrument Performan e . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Issues that a e t performan e . . . . . . . . . . . . . . . . . . . . . . . . .

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ii

CONTENTS

3.4 Image re onstru tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.4.1 Sele ting the sub- ollimators and energy range . . . . . . . . . . . . . . . 26
3.4.2 Testing the Image Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

A Useful HESSI Software

29

B Web Version of the HAG

29

LIST OF FIGURES

iii

List of Figures
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8

Outline of HESSI Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Energy resolution of HESSI as a fun tion of photon energy. . . . . . . . . . . . .
HESSI grid modulation amplitudes. . . . . . . . . . . . . . . . . . . . . . . . . .
HESSI angular overage as a fun tion of photon energy. . . . . . . . . . . . . . .
Outline of the Germanium Dete tor used on HESSI . . . . . . . . . . . . . . . .
Attenuator transmission as a fun tion of energy . . . . . . . . . . . . . . . . . . .
HESSI total e e tive areas for the front and rear dete tor segments. . . . . . . .
Demonstration of how the modulation hanges with sour e position, intensity
and size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15
17
18
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20
21
24
26

iv

LIST OF FIGURES

1

1

Introdu tion

This is the HESSI Analysis Guide (HAG). The Guide is in two main parts, the HESSI Users
Guide (Se tion 2) and the HESSI Instrument Guide (Se tion 3). The User Guide des ribes how
to analyse HESSI data and the Instrument Guide dis usses aspe t of the HESSI instrument.
Additional information is provided in the Appendi es.
1.1

The HESSI Mission

The HESSI mission onsists of a single spin-stabilized spa e raft in a 600km orbit in lined at
38Æ to the Earth's equator. The only instrument on board is an imaging spe trometer withthe ability to obtain high delity movies of solar ares in X-rays and -rays. It uses two
new omplementary te hnologies: ne grids to modulate the solar radiation, and germanium
dete tors to measure the energy of ea h photon very pre isely.
HESSI's full Sun imaging apability is a hieved with grids that modulate the solar X-ray
ux as the spa e raft rotates at 15 rpm; up to 20 images an be obtained per se ond. The
high-resolution spe tros opy uses germanium rystals that dete t the X-ray and -ray photons
transmitted through the grids over the broad energy range from 3 keV to 17 MeV.
HESSI will be laun hed in June 2001 on a Pegasus (SELVS-II) laun h vehi le. The hoped forbaseline mission duration is 3 years. The low-altitude equatorial orbit was hosen to minimize
damage to the germanium dete tors from the harged parti les in the Earth's radiation belts.
The orbit's in lination was hosen so that the ground-station at University of California at
Berkeley (UCB) an ommand the spa e raft and re eive telemetry for 5 orbits/day.
1.2

HESSI Instrument Data

Telemetry pa kets downlinked from the HESSI spa e raft are saved in a number of les withnames that orrespond to the time of the onta t initiation. There are three types of les:spa e raft housekeeping (VC1), real time s ien e (VC2), and event list (VC3). The VC1 andVC3 pa kets are used to produ e the reformatted HESSI instrument data { VC2 pa kets, whi hshould be a subset of VC3, are only used during the real-time onta ts.
The HESSI ar hive data are reformatted into les orresponding to one orbit's worth of data.
The les are in FITS format and have names of the form \hsi yyyymmdd hhmmss nnn. ts",
where the time yyyymmdd hhmmss refers to date and UT for the rst pa ket of the le, andnnn is a version number (used if the le is regenerated).
HESSI data largely onsists of event lists; these in lude the energy and arrival time of ea h
event, together with information about the spa e raft aspe t and rotation. The size of an orbit
le depends on the dete tors ountrates, but they are typi ally a few tens of megabytes in size.
Files whi h are larger than 50 Mb are broken into smaller les of 50 Mb or less { this should
only happen for very large ares. Normally, unless there are multiple les in an orbit, the start
time of a le is the middle of spa e raft night { for multiple les, a new le is reated for every
30 thousand pa kets, ea h with a orresponding start time. Approximately 1 Gb of data areprodu ed by HESSI ea h day. At 4 byte/photon, this equates to 200 M photons/day, most of
whi h are ba kground photons, ex ept during very large ares.
After reformatting, the ar hived data are used to generate the HESSI Data Catalog { this inludes the Observing Summary, Flare List and Instrument Log. The Data Catalog is in the formof les that over a whole day and have names of the form hessi atalog yyyymmdd. ts.
The part of the data atalog for the time interval overed by the FITS le is in luded in the le.

1 INTRODUCTION

2

The omplete atalog for the entire mission is distributed within the SSWDB (SSW DataBase)tree; it is also kept online on an SOC workstation at UCB and is available for browsing throughthe Web.
The whole-mission are list (hessi are list. ts), and a le database (hsi ledb. ts)
that ontains information relating to all les reated in reformatting, are distributed within the
HESSI bran h of the SolarSoft (SSW) tree.
The ar hived data will be available through the SDAC (Solar Data Analysis Center) at
NASA GSFC, and through the HEDC (HESSI Experimental Data Center) at ETH, Zuri h.
1.3

Referen es

The High Energy Solar Spe tros opi Imager (HESSI); 15 June 1997, proposal in

response to NASA AO 97-OSS-03 (UCBSSL 1925/97)
The High Energy Solar Spe tros opi Imager (HESSI) - As Built; 6 Feb 2001, Brian Dennis,
GSFC.
Behind, Beneath, and Before HESSI Spe tros opy; 4 Feb 2001, David Smith, UCB.
1.4

Conta ts

Bob Lin
Gordon Hurford
David Smith
Jim M Tiernan
Andre Csillaghy
Chris Johns-Krull(SSL, UCB)
(SSL, UCB)
(SSL, UCB)
(SSL, UCB)
(SSL, UCB)
(SSL, UCB)

ghurfordapollo.ssl.berkeley.edu
dsmithssl.berkeley.edu
jimmssl.berkeley.edu
sillagssl.berkeley.edu
mjssl.berkeley.edu

Brian Dennis
Ri hard S hwartz
Kim Tolbert
Ed S hmahl(GSFC)(GSFC)(GSFC)(GSFC)

Brian.R.Dennisgsf .nasa.gov
ri hard.s hwartzgsf .nasa.gov
Anne.K.Tolbertgsf .nasa.gov
s hmahlhesperia.gsf .nasa.gov

Also see the HESSI Team web page.
1.5

A knowledgements

The HESSI Analysis Guide (HAG) was prepared by Bob Bentley (rdbmssl.u l.a .uk) of
the Mullard Spa e S ien e Laboratory, with signi ant help and input from Ri hard S hwartz,
Andre Csillaghy, Kim Tolbert, Domini Zarro, Chris Johns-Krull, and many other members ofthe HESSI Team.

3

2

HESSI User Guide

To run the HESSI analysis software, you need an IDL installation of version 5.3 or higher.
When starting SolarSoft, you should in lude hessi, and the spex and xray pa kages. See theUCB Data Analysis Software and SolarSoft do umentation web sites for more information.
In an attempt at sour e ontrol, HESSI software is available both at its \normal" lo ationin the SSW tree ($SSW HESSI/idl), and in a release dire tory ($SSW HESSI/release/idl). The
most re ent version of the software is in $SSW HESSI/idl, while the release dire tory ontains
software frozen at a parti ular time. You an swit h between the release and development
version of the HESSI software using hessi version { this should only be alled at the beginningof an IDL session sin e it does not re ompile routines { by default you should be using thedevelopment version (???) .
hessi_version, /dev
hessi_version, /release

The HESSI analysis ode is obje t oriented. A list of the obje ts used is given on the
UCB Web pages (http://hessi.ssl.berkeley.edu/software/referen e.html). Sin e obje t oriented
ode may be new to some users, a Tutorial on obje ts has been prepared by Andre Csillaghy{ see the Web page on URL http://hessi.ssl.berkeley.edu/software/hessi oo on ept.html.
The HESSI software has memory - this allows the user to set up and then reuse parameters,
but does require that some parameters be reset when a di erent option is required.
It is possible to run a lot of the HESSI software through the HESSI Graphi al User
Interfa e (GUI). This allows a ess to mu h of the software through pull-down menues andbuttons. A brief summary of this is given in Se tion 2.8.

2.0.1 Environment Variables
The exe ution of the HESSI analysis ode depends on a number of environment variables thatde ne the lo ation of the HESSI data, observing summary and are atalogs. The HESSI
data les are under HSI DATA ARCHIVE, the HESSI atalog, are list and le database under HSI CATALOG ARCHIVE, HSI FLARE LIST ARCHIVE and HSI FILEDB ARCHIVE
respe tively, and the spa e raft ephemeris is under HSI PREDICTED DIR. Default values forthese are spe i ed by the experiment team in the setup.hessi env lo ated in $SSW HESSI/setup.
These an be altered by the user by pla ing a modi ed version of the le in the site setup dire tory ($SSW/site/setup).
2.1

Sele ting and Reading Data

2.1.1 The HESSI Observing Summary
The observing summary data are obtained dire tly from the telemetry pa kets and the les
allow the user to look at HESSI outrates and other information before a essing the data les.
The photons are time-binned and the ount rate is ompressed to bytes (0.03% a ura y
up to 1.0e6 ounts/se ). Modulation varian e is al ulated for the 2 oarsest ollimators by
al ulating the standard deviation of ounts for the given 4 se interval and dividing by thesquare root of the total ounts in that interval. The spa e raft position (ephemeris) and datafrom the parti le dete tor are also in luded in the les.
Information an be extra ted using the plot and list methods:

2 HESSI USER GUIDE

4

obs_summ = obj_new('hsi_obs_summary') ; reating an instan e of the lasstime_interval = ['01-Sep-00 12:00', '01-Sep-00 14:30'℄
ounts = obs_summ -> getdata(obs_time_interval=time_interval)orounts = obs_summ -> getdata(filename='hsi_20000830_235100_001.fits')
obs_summ -> plot
text = obs_summ -> list();default to ountrates

Options to the plot methods in lude showing are, e lipse, and SAA intervals, plottingenergy bands separately or summed, sele ting energy band range to plot, and plotting witholors. Also, any standard plot keywords an be passed to the plot method. If you want olor
plots, you must set up the olors rst, e.g. by alling line olors. (?????? problem under UNIX)
obs_summ -> plot
;default to ountrates
obs_summ -> plot, / ountrate
obs_summ -> plot, /parti le_rate
obs_summ -> plot, /ephemeris
obs_summ -> plot, /mod_varian eounts = obs_summ -> getdata( lass_name='obs_summ_rate')

Options to the list methods in lude showing the list on the s reen, saving the list in a le,and printing the list.
text = obs_summ -> list();default to ountrates
text = obs_summ -> list(/parti le_rate)
text = obs_summ -> list(/ephemeris)text = obs_summ -> list (/show, /print, /file_text)
text = obs_summ -> list (/ephem, file='ephem.txt')

It is also possible to plot the ountrate against time using utplot. To do this, you need to
extra t the time seperately:ounts = obs_summ->getdata()
times = anytim(obs_summ->getdata(/time),/ints)
utplot,times, ounts(2,*)

2.1.2 Using the HESSI Flare List
The HESSI are list an be retrieved using the fun tion hsi read arelist, whi h returns astru ture of all the ares re orded to date.
flist = hsi_read_flarelist()

The are list stru ture provides all sorts of information that is useful in sele ting HESSI
data: the start, peak and end time of the are, the peak ountrate, the are lo ation andthe name of the FITS le that ontains the are data. We an examine the ontents of thestru ture, by:5

2.1 Sele ting and Reading Data
IDL> help,/st,flist(0)** Stru ture HSI_FLARELISTDATA_EXT, 15 tags, length=144:
ID_NUMBER
LONG
1365
START_TIMEDOUBLE
6.8368384e+08
END_TIMEDOUBLE
6.8368386e+08
PEAK_TIMEDOUBLE
6.8368384e+08
BCK_TIMEDOUBLE
Array[2℄
ENERGY_HI
FLOAT
Array[2℄PEAK_COUNTRATE FLOAT
880.000
BCK_COUNTRATE FLOAT
59.6774
TOTAL_COUNTS
FLOAT
8960.00
POSITIONFLOATArray[2℄FILENAMESTRING'hsi_20000830_235100_001.fits'
FLAGS
BYTE
Array[32℄ACTIVE_REGION INT
0
X_POSITION
FLOAT-892.500
Y_POSITION
FLOAT
507.500
IDL>

If you wish to sele t a subset of the ares in the HESSI are atalog, this an be done witheither hsi sele t are or hsi are at. The hsi sele t are fun tion allows the sele tion to
be made using keywords that orrespond to the tags in the are list stru ture , while
hsi are at provides a GUI interfa e to make the sele tion. In both ases, the results of thesele tion an be returned as either a list of are IDs, a formatted list of the information relating
to the are, or a are list stru ture.
; return formatted list of flares - sear h by range in keyword
flist = hsi_sele t_flare(id=[1390,1396℄)
flist = hsi_sele t_flare(start=time_interval, /form)
flist = hsi_sele t_flare(peak_ ountrate=[1500,2000℄, /stru ture)
hsi_flare at, list=flist
hsi_flare at, list=flist, /form
hsi_flare at, list=flist, /stru ture

; return list of flare numbers; return a formatted list
; return a stru ture

The are times and position given in the are list struture an be used to read the instrument
data. In the example shown, the le starts at 23:51 UT on 30-Aug-00; the start and end timesof the are are 31-Aug-00 00:10:40 and 31-Aug-00 00:10:56, whi h an be determined as shown
below; the are position (in ar se onds in helio entri oordinates) is given in the position tag.
These an be fed into the HESSI analysis software, e.g. hsi image or hessi image using the
obs time interval and position keywords (see Se tion 2.3).
t0 = anytim(flist(0).start_time,/yohkoh)
t1 = anytim(flist(0).end_time,/yohkoh)
time_interval = [t0,t1℄
position = flist(0).positiono_img = hsi_image(); reate an instan e of the lassim = o_img -> getdata(obs_time_interval=time_interval, $xyoffset=position)

2 HESSI USER GUIDE

6

2.1.3 Using a Map Obje t or an SSW Index Re ord
When re onstru ting a HESSI image, you need a time range and the oordinates of the aresite. In the se tions above, these were derived from the HESSI Observing Summary and Flare
List. They an also be derived from SSW produ ts produ ed when analysing data from othersour es, e.g. SXT or TRACE. A \map obje t" produ ed by the Mapping software of Domini
Zarro, or an SSW- omplient index stru ture an both provide the ne essary information (seeSe tion 2.6.1).
Assuming you have already made a map obje t, or have a set of index stru tures, pro eed asshown below. Note that the HESSI software needs a time interval when sele ting observations- for map obje ts, the single time needs to be onverted into an interval.
position = [map.x ,map.y ℄; using Map stru ture
time_window, map.time, t0,t1, se =[0,300℄ ; need a time interval
time_interval = [t0,t1℄; (maps only have single time)
position = [index.x en,index.y en℄
time_window, index, t0,t1
time_interval = [t0,t1℄
2.2; using SSW- omplient index; retrieve start and stop times

Time plots of HESSI Data

Start by reating an instan e of the light urve lass of obje ts, then plot an integrated lighturve (with a 1 se . resolution) for a sele ted time interval:
o_lt = HSI_Light urve(); reating an instan e of the lasstime_interval = ['2000/09/01 12:00', '2000/09/01 14:00'℄
o_lt -> Plot, obs_time_interval=time_interval, lt _time_resolution=1

2.2.1 Retrieving parameters
On e the data has been ingested, it is also possible to retrieve arrays of required parameters.
The all:taxis = o_lt -> GetData( /xaxis )returns the start times of the time bins. While:lt = o_lt -> GetData( time_range=[0,60℄, $lt _energy_band=[3,100,500,1000,15000℄ )will return a 2D array lled with the ounts a umulated in ea h time bin for ea h energy
band. In this example, the time range of the light urve orresponds to the rst 60 se onds inthe sele ted time range and thus a 604 array is returned.
Light urves for sele ted dete tors (in the range 1{9) or segments (in the range 1{18) an be
retrieved with ommands like:
lt = o_lt -> GetData( this_det = [1,2℄ )lt = o_lt -> GetData( this_seg = [1,10℄ );dete tors 1 and 2;segments 1 and 10

7

2.3 Image Re onstru tion

You an plot any extra ted parameter against time (taxis ) using utplot:
utplot, taxis, lt

Coin ident events (see Se tion 3.3.2) an ause problems with ????, and you my want tolook at them:s = o_lt -> GetData( / oin iden e )returns a 4-dim array where the 4th dimension ontains the anti- oin ident light urve (index#0) and the oin ident light urve (index #1)
2.3

Image Re onstru tion

In order to re onstru t an image, ertain pie es of information need to be supplied. Thesein lude the start time and \integration" time of the image, the enter (in helio entri oordinates) that the image should be re onstru ted about, the image algorithm... ??? fov, energy
range...
Note that the HESSI software needs a time interval when sele ting observations, but ashorter interval (de ned in time range ) should be used for image re onstru tion.
After reating an instan e of the image lass of obje ts, you an list the parameters of theobje t using the \print" method.
o_img = hsi_image()o_img -> print; reate an instan e of the image lass; of obje ts; list obje t parameters

time_interval = ['2000/09/01 12:00', '2000/09/01 14:00'℄
position = [200,400℄; x and y enter, in heliographi; (ar se s from sun enter)im = o_img -> getdata(obs_time=time_interval,
time_range=[0,10℄,
xyoffset=position
image_algorithm=' lean')o_img -> plot

oordinates; overall interval; start, stop se onds from; start of overall interval; enter for re onstru tion
; re onstru tion algorithm,; ba k-proje tion by default; plot the image

2.3.1 Re onstru tion Te hniques
Currently, there are ve di erent image re onstru tion algorithms available in the HESSI software: Ba k Proje tion plus Clean, MEM-Sato, MEM-Vis, Pixon, and Forward Fitting. These
are all implemented within hessi image and hsi image { the default is Ba k Proje tion.
1. Ba k Proje tion (plus Clean). The default image re onstru tion algorithm is ba kproje tion with lean disabled. Ba k proje tion simply operates by multiplying the alibrated
event list by the ollimator modulation patterns to onstru t a dirty map of the image. The
map an then be improved by using Clean - this is sele ted by passing \ lean" into the image algorithm keyword.

2 HESSI USER GUIDE8
o_img = hsi_image(); reate an instan e of the lassim = o_img -> getdata(obs_time=time_interval, $; default no Clean
time_range=[0,10℄, xyoffset=position);orim = o_img -> getdata(obs_time=time_interval, image_algorithm=' lean')o_img -> plot

To generate a default model image use:
im = o_img -> getdata()
o_img -> plot

2. MEM-Sato is an image re onstru tion algorithm utilizing the maximum entropy method(MEM) des ribed by Sato, Kosugi and Makishima (1999, PASJ, 51, 127-150). In general, MEMSato gives very sharp images, but is time onsuming to run. This is espe ially true if all theollimators are sele ted for the imaging. The nest four ollimators (0 - 3) require substantial
omputing time sin e they have the most rapid modulations and hen e require the nest timebins. Eliminating these ollimators from the al ulation will ause a loss of resolution, butwill dramati ally in rease the speed of the image re onstru tion. This is done through thedet index mask keyword. For example, if you wish to use MEM-Sato to onstru t an image
using only ollimators 5 - 9 (indi es 4 - 8), you would type:o_img = hsi_image()
im = o_img -> getdata(obs_time=time_interval,image_algorithm='mem sato', $det_index_mask=[0,0,0,0,1,1,1,1,1℄)o_img -> plot

If you wish to use MEM-Sato to onstru t an image of the default model using the sameollimators, enter:o_img = hsi_image()
im = o_img -> getdata(/plot,image_algorithm='mem sato', $det_index_mask=[0,0,0,0,1,1,1,1,1℄)

3. MEM-Vis is an image re onstru tion algorithm borrowed from radio astronomy te hniques - see Conway et al., 2001, in preparation ???.
The MEM-Vis algorithm uses the same basi steps as MEM-Sato, but works in polar oordinates and uses \visibilities"1 rather than \ ounts". In the HESSI ontext, ounts andvisibilities an simply be thought of as two time series representing the same information: theresponse of HESSI to an image. Whereas ounts are non-negative integer values, visabilities areomplex numbers (although the real and imaginary parts do not ontain independent information). Sin e aspe t orre tion an easily be applied when onstru ton visibilities from observed
ounts, visibility modulation patterns do not need to be orre ted for aspe t for ea h timebin and visibility values over several rotations an be simply added together. In onstru ting
1

The word \visibility" was oined by radio astronomers, and has rept into the lexi on of X-ray astronomers
by way of the Rotation Modulation Collimator (RMC), whi h is the equivalent of Earth Rotation Synthesis inradio astronomy. The bridge between radio and X-ray terminology is the modulation pattern. For an X-ray
teles ope, this is the two-dimensional eld of probabilities that a photon oming from a ertain dire tion willpass through the rotating ollimator to the dete tor.

2.3 Image Re onstru tion

9

visibilites, there is e e tively a smoothing. This relaxes problems arising from the presen e ofzero ounts, but introdu es a small systemati error and a orrelation of noise on visibilities in
the neighbouring time bins.
So, whereas MEM-Sato an be slow for a long are, MEM-Vis does not take mu h longer
than for a short are sin e visibilities an be simply added together over multiple rotations.
o_img = hsi_image()
im = o_img -> getdata(obs_time=time_interval, image_algorithm='vis')o_img -> plot

4. Pixon-based image re onstru tion tries to maximize the image resolution in lo ations
where the sour e intensity is high, but permits the resolution to be low where the sour eintensity is low. Thus, the \pixels" in a pixon ode an vary in shape and size a ross the image.
The method used here is the fra tal pixon method of Puetter and Pina (1994, Experimental
Astronomy, 3, 293: see also Pina and Puetter 1993, PASP, 105, 630 and Met alf et al. 1996,
ApJ, 466, 585). This is a very powerful method, giving good noise suppression and goodre overed photometry, but is very time onsuming. It is suggested that users hoose a di erent
te hnique to de ide the key issues of time overage, energy range, et . for their images, andthen use the pixon method to reate nal images for analysis and display.
o_img = hsi_image()
im = o_img -> getdata(obs_time=time_interval, image_algorithm='pixon')
o_img -> plot

5. Forward Fitting assumes that the sour e an be represented by a number of superimposed 2D gaussians, whi h an be either i ular, ellipti al, or urved. The algorithm adjusts
the modulation patterns al ulated from a model map ontaining multiple Gaussian sour es tothe alibrated event list of the telemetry le. The primary advantages of the forward tting
algorithm is its speed and relatively robust estimation of the sour e parameters. The urrent
implementation allows model- tting by an arbitrary number of 2D Gaussian sour es. The initial default value is one single sour e (n gaussians =1). If the resulting hi-square is near unity,
no re nement of the model is justi ed. However, if the resulting redu ed hi-square ex eeds thevalue of 1.1, the user should in rease the number of gaussian omponents (e.g n gaussians =2),
until the model is onsistent with a hi-square near unity. Be ause the forward- tting algorithm
only onverges well if the initial guess of the sour e positions is a urate, losely spa ed sour es
an ause a problem.
??? obje t ode
hessi_image,im,par,/plot,image_algorithm='forward'
hessi_image,im,par,/plot,image_algorithm='forward',n_gauss=2

A detailed des ription of the algorithm and its testing an be found in As hwanden et al.
2001 (ApJ, in preparation). Additional information on the forward tting method an befound at the Lo kheed-Martin web site.

2 HESSI USER GUIDE

10

2.3.2 Verifying the Image quality
Given the potential problems with image re onstru tion (see Se tion 3.4), below are some re ommendations for testing the reality of features in your image:
1) Re-image the sour e with:

 A di erent imaging algorithm A di erent eld of view whi h still en ompasses the sour e A di erent pixel size
 A di erent ombination of sub ollimators
 A di erent energy range
 A di erent time range
 A di erent set of alibration parameters Odd/even half-rotations
2) Apply redundan y tests:

 Compare spe tra/light urves from di erent dete tors
 Che k rotational symmetry of light urve3) Tra e ba k the feature to: Fourier omponents
 Observed modulation urves4) Simulate and analyse similar sour es
These onsiderations also reveal the importan e of being able to simulate HESSI data (seese tion 2.9). The hessi image software o ers a fairly exible means to ontru t simulated data
whi h an then be analyzed to see how the imaging software re overs what was in luded in themodel.

2.3.3 Keywords related to Imaging
Under onstru tion!!! Below, we summarize the keywords that are most important for image
re onstru tion.
A omplete list of keywords is given...
2.4

Spe tros opy

Start by reating an instan e of the spe trum lass of obje ts, then de ne the time interval ofinterest, and energy range and binning, et . before retrieving the data:o_sp = hsi_spe trum(); reate an instan e of the lasstime_interval = '31-aug-00 '+['11:40:00','12:00:00'℄
o_sp -> set, time_range=[0,300.℄, obs_time_interval=time_interval, $sp_energy_binning=1, sp_time_interval=2.0, energy_band=[0.,0.℄spe = o_sp -> getdata(); retrieve ount-rate spe trum

2.5 More Complex Data produ ts

11

The keyword sp energy binning sele ts les ontaining prede ned energy bin ranges { in theexample, the sp energy binning=1 sele ts the le \ t edges.1 ", from spe resp dire tory in the
HESSI bran h of the $SSWDB tree. Be ause several parameters need to be de ned, the set
method was used { it would also have been possible to pass these as parameters in the getdata
method.
As with imaging, the this det and this seg keywords an be used to return only the spe trum
for the desired dete tors or segments.
spe = o_sp -> getdata( this_det=[1,2℄ )spe = o_sp -> getdata( this_seg=[1,10℄ )The average spe trum an be plotted on a log-log s ale using:plot, o_sp -> getdata(/xaxis,/mean), total(spe ,2), psym=10,/xlog,/ylog

To retrieve and plot a alibrated spe trum, use:
spe = o_sp -> getdata(/sp_semi_ alibrated)
plot, o_sp -> getdata(/xaxis,/mean), total(spe ,2), psym=10,/xlog,/ylog

The boundaries of the energy and time bins used in produ ing the spe tra an be retrieved
using:
energy_edge = o_sp -> getdata(/xaxis, /edges_2)
ut_edge = o_sp -> getdata(/yaxis, /edges_2)

The pro edure hsi spe tros opy list lists information on HESSI spe tros opy.

2.4.1 Keywords related to Spe tros opy
Under onstru tion!!! Below, we summarize the keywords that are most important for spe troopy.
A omplete list of keywords is given...
2.5

More Complex Data produ ts

2.5.1 Imaging Spe tros opy
Under onstru tion!!! See Ri hard S hwartz.
Sets of images of di erent spe tral ranges for the same time interval. Currently needs to bedone by repeated all to hsi image.
maps of spe tral hardness; temperature maps?; density?

2.5.2 Image Cube Light urves
Under onstru tion!!!
Light- urves of features in image (from image CUBES) Currently needs to be done byrepeated all to hsi image.

2 HESSI USER GUIDE

12
2.6

Comparing HESSI with Other Data

2.6.1 Comparing with Images
When re onstru ting a HESSI image, you need a time range and the oordinates of the are site.
These an be derived from SSW produ ts produ ed when analysing data from other sour es,
e.g. SXT or TRACE.
A \map obje t" (produ ed by the Mapping software of Domini Zarro), or an SSWomplient index stru ture, an both provide the ne essary information. (Note: The TRACE
index stru ture is SSW- omplient, but sin e the SXT index stores the image entre in terms of
CCD pixel oordinates rather than helio entri oordinates, it is ne essary to onvert the SXT
index stru ture to an SSW- ompliant stru ture using stru t2ssw.)
Assuming you have already made a map obje t, or have a set of index stru tures, pro eed asshown below. Note that the HESSI software needs a time interval when sele ting observations,but a shorter interval (de ned in time range ) should be used for image re onstru tion.
position = [map.x ,map.y ℄time_interval = anytim(map.time)+[0,300℄; using Map stru ture
; need a time interval;orposition = [index.x en,index.y en℄
fmt_timer,index,t0,t1
time_interval = [t0,t1℄; using SSW index stru ture
; retrieve start and stop times

o_img = hsi_image()
im = o_img -> getdata(obs_time_interval=time_interval, $time_range=[0,10℄, xyoffset=position)
hmap = o_img -> makemap(); make a map obje t
plot_map, map
plot_map, hmap,/over; plot original data
; overplot HESSI

See the SSW Mapping tutorial, and Tra e Analysis Guide for more information.!!
http://www.lmsal. om/solarsoft/sswdo /solarsoft/ssw_eit_mdi.html
http://www.lmsal. om/solarsoft/sswdo /guides/tag/

Mention also the an illary databases (se tion 2.7...

2.6.2 Comparing Time seriesAlso, ompare in time...
2.7

HESSI Synopti Data Ar hive

The HESSI Synopti Data Ar hive is an an illary database ontaining observations fromground-based and other spa e-based observatories that are useful in supporting the HESSI observations. The ar hive is an extension of the SoHO Summary and Synopti Database used
to aid planning for the SoHO mission; urrently it onsists only of image data and light urves.
You an a ess the HESSI Synopti Data Ar hive through SolarSoft. This is available asan option in the HESSI GUI (se tion 2.8), or by using the show synop GUI.

2.8 HESSI Graphi User Interfa e (GUI)13

show_synop

The GUI allows the user to de ne a data sour e and time range. The user is presented witha list of les that mat h those riteria. After making a sele tion, the FITS les ontaining therequested images are then retrieved and stored on the users omputer.
Another onne ted database is sear h engine implemented as part of the Max Millennium Program. This sear hes primarily on data taken during ampaigns, or are events.
2.8

HESSI Graphi User Interfa e (GUI)The Graphi al User Interfa e to the HESSI data analysis software is an IDL widget programthat allows users to display, analyze, and store HESSI image, spe tros opy, light urve, andhousekeeping data, as well as view data from an illary sour es.
All of the fun tionality available through the GUI exists at the ommand line level of IDL.
The ommand line interfa e requires some knowledge of IDL and HESSI-spe i pro edure andkeyword names, but allows maximum exibility and ontrol, while the GUI should be easier touse.
The GUI interfa e is invoked by typing:hessi

Additional information on the HESSI GUI is available on the Web at the UCB site { see
URL http://hessi.ssl.berkeley.edu/software/hessi gui.html.
more???...
2.9

Simulating HESSI Data

One way of verifying that the image re onstru tion te hniques have produ ed a good result is
to try to simulate the input image. A sophisti ated tool is being developed, but it is possible touse hessi image for the task. The hessi image simulates data when supplied with a modelthrough the sim model keyword. Two types of models an be de ned: an array of stru turesde ning Gaussian sour e hara teristi s, or a 2D array ontaining point sour es.
In the rst method, it is ne essary to de ne values in a stru ture de ned for ea h Gaussian
sour e, then fold the (one or more) sour es together in an array before running the model:
sr 1 = {gaussian_sour e_str} ; reate sour e stru ture and populate itsr 1.amplitude = 1.0; intensity of sour esr 1.xypos = [10.,2.℄; x and y offsets (ar se ) from image entersr 1.xysigma = [5.,2.℄; x and y standard deviation (ar se )sr 1.tilt_angle_deg = 0.
; tilt angle (degrees) of ellipsoid from; x-axis, in an anti- lo kwise dire tionsr 2 = {gaussian_sour e_str}sr 2.amplitude = 0.5
sr 2.xypos = [20.,-3.℄
sr 2.xysigma = [5.,2.℄sr 2.tilt_angle_deg = 0.
model = [sr 1,sr 2℄; make an array of the sour e definitions
o_img = hsi_image(); reate an instan e of the lassim = o_img -> getdata(sim_model=model) ; simulate using supplied model2 HESSI USER GUIDE

14
o_img -> plot

; default image size is 64x64

The ommand line format for the last three lines would be:hessi_image,sim_model=model,/plot

In the se ond method, a 2D array is reated and point sour es of the desired strength arepla ed within the array:model = fltarr(64,64)
model[10,20℄ = 1.0
model[30,12℄ = 0.5
model[15,40℄ = 1.0; sour e strength half of the otherso_img = hsi_image(); reate an instan e of the lassim = o_img -> getdata(sim_model=model, /plot)

By default, the simulated data will be onstru ted assuming 1000 photons per se ond perollimator in ident on the spa e raft. The number of photons needs to be in reased to improve
the statisti s if there is more than one sour e. The number of photons in the simulation an bede ned using the sim photons per oll keyword:
o_img = hsi_image()
im = o_img -> getdata(sim_model=model,sim_photons_per_ oll=5000)
o_img -> plot

Eventually, the hessi sim are pro edure will be available... See the des ription ofhessi sim are on Jim M Tiernan Web site.
hessi_sim_flare
2.10

Software Help

Under onstru tion!!!

15

Figure 3.1: Outline of HESSI Instrument

3

HESSI Instrument Guide

The Instrument Guide is intended to give the user an outline of the HESSI observatory, andto onvey some of the salient points relating to the instrument that may be of assistan e whenanalysing the data. It is by no means the only sour e of this information. Other referen es???
The Web version of this do ument ontains links to pages with additional information.
3.1

The HESSI Observatory

Below is an outline of how the HESSI Observatory works. More information about the HESSI
instrument and spa e raft an be found on the UCB Web pages (http://hessi.ssl.berkeley.edu).

3.1.1 Instrument Des ription

The HESSI mission onsists of a single spin-stabilized spa e raft in a 600km orbit in lined at 38Æ
to the Earth's equator. The only instrument on board is an imaging spe trometer (Figure 3.1)
with the ability to obtain high delity movies of solar ares in X-rays and -rays. It uses two
new omplementary te hnologies: ne grids to modulate the solar radiation, and germanium
dete tors to measure the energy of ea h photon very pre isely.

16

3 HESSI INSTRUMENT GUIDEHESSI's imaging apability is a hieved with ne tungsten and/or gold grids that modulate
the solar X-ray ux as the spa e raft rotates at 15 rpm. Up to 20 images an be obtained
per se ond. This is suÆ ient to tra k the ele trons as they travel from their a eleration site,
believed to be in the solar orona, and slow down on their way to the lower solar atmosphere.
The high-resolution spe tros opy is a hieved with 9 ooled germanium rystals that dete t
the X-ray and -ray photons transmitted through the grids over the broad energy range of 3
keV to 20 MeV. Their ne energy resolution of about 1 keV is more than suÆ ient to reveal
the detailed features of the X-ray and -ray spe tra, lues to the nature of the ele tron and ion
a eleration pro esses.
A spinning spa e raft pointing at or near Sun enter provides a simple and reliable way
to a hieve the rotation required for the HESSI imaging te hnique. The low-altitude equatorial
orbit that an be rea hed with a Pegasus laun h vehi le is hosen to minimize damage to thegermanium dete tors from the harged parti les in the Earth's radiation belts.
The HESSI s ienti obje tives will be a hieved with high resolution imaging spe tros opy
observations from soft X-rays to -rays, utilizing a single instrument onsisting of an Imaging
System, a Spe trometer, and the Instrument Ele troni s.
The Imaging System is made up of nine Rotating Modulation Collimators (RMCs), ea honsisting of a pair of widely separated grids mounted on a rotating spa e raft. Pointing
information is provided by the Solar Aspe t System (SAS) and Roll Angle System (RAS)The Spe trometer has nine segmented GeDs, one behind ea h RMC, to dete t photons from
3 keV to 20 MeV. The GeDs are ooled to <75 K by a spa e-quali ed long-life me hani al
ryo ooler to a hieve the highest spe tral resolution of any presently available -ray dete tor. As
the spa e raft rotates, the RMCs onvert the spatial information from the sour e into temporalmodulation of the photon ounting rates of the GeDs.
The energy and arrival time of every photon, together with SAS and RAS data, are re ordedin the spa e raft's on-board 2-Gbyte solid-state memory (sized to hold all the data from thelargest are) and automati ally telemetered within 48 hours. With these data, the X-ray and-ray images an be re onstru ted on the ground The instrument's 1Æ eld of view is mu hwider than the 0.5Æ solar diameter, so all ares are dete ted, and pointing an be automated.
The enery resolution of HESSI (as a fun tion of photon energy) is shown in Figure 3.2. For
omparison, the resolution of instruments from on previous missions is also shown. HESSI hassigni antly better resolution than any previous mission.

3.1.2 Spa e raft Pointing
HESSI is a Sun pointing spa e raft and is spin stabilized. Rotating at 15 rpm, the spa e raftspin axis is pointed within 12 ar se s of Sun entre { the spin rate stability is <4.5 ar se onds
per se ond. The spa e raft inertia an be adjusted using the solar panels to align the imager
with the spin axis. Other than this, magneti torque rods are used for all pointing ontrol.
After laun h, the spa eraft is able to a quire the Sun and spin up from any orientation without
ground intervention.
HESSI observes the whole Sun and images are built up from Fourier omponents produ ed
when the rotating ollimator modulates a sour e. Images are re onstru ted around a desired
entre { to do this, it is essential to know a urately the pointing of the spa e raft rotation axis
and the phase of rotation. This information is provided by the Solar Aspe t Sensor (SAS) andRoll Angle Sensor (RAS) { see se tions 3.2.4 and 3.2.5.

3.2 HESSI Instrument Subsystems

17

Figure 3.2: Energy resolution of HESSI as a fun tion of photon energy.
3.2

HESSI Instrument Subsystems

3.2.1 Imaging System
The imaging apability of HESSI is based on a Fourier-transform te hnique using a set of 9
Rotational Modulation Collimators (RMCs). Ea h RMC onsist of two widely-spa ed, ne-s ale
linear grids, whi h temporally modulate the photon signal from sour es in the eld of view as
the spa e raft rotates about an axis parallel to the long axis of the RMC. The modulation(see Figure 3.3) is measured with a dete tor having no spatial resolution pla ed behind the
RMC. The modulation pattern over half a rotation for a single RMC provides the amplitude
and phase of many spatial Fourier omponents over a full range of angular orientations but
for a small range of spatial sour e dimensions. Multiple RMCs, ea h with di erent slit widths,
provide overage over a full range of are sour e sizes. An image is re onstru ted from theset of measured Fourier omponents in exa t mathemati al analogy to multi-baseline radio
interferometry.
HESSI provides spatial resolution of 2 ar se onds at X-ray energies below 40 keV, 7
ar se onds to 400 keV, and 36 ar se onds for -ray lines and ontinuum above 1 MeV (seeFigure 3.4). The hosen spa e raft rotation rate of 15 rpm provides a omplete image withthe maximum number of Fourier omponents in 2 s, but spatial information from fewer Fourier
omponents is still available on time s ales down to 10's of ms, provided the ount rates aresuÆ iently high.
For HESSI, the separation between grids in ea h RMC is 1.55 m and the grid pit hes range

3 HESSI INSTRUMENT GUIDE

18

1.0

Grid
#2

Grid
#9

Relative Modulation Amplitude

0.8

#6
0.6

Diffraction Limited

Transmission Limited#5

0.4#4,7,&8

#1 #2

0.2

#3
0.0
1

10

100
Energy (keV)

1000

10000

Figure 3.3: HESSI grid modulation amplitudes.

Angular Coverage vs. Photon Energy

Sun’s Radius

1000

Yohkoh/SXTHESSI SMEX(~1 keV resolution)

Yohkoh/HXT(4 energy bins)
Angular Coverage (arcsec)

TypicalDimensions__________

100

Flare
ExtentFootpoint
Separation

10

Distance traveled byelectron in 50 ms
Footpoint
Diameter

1
1 keV

10 keV

Thermal Plasma (Superhot)

100 keV
Energy

1 MeV

10 MeV

Gamma-ray lines
Nonthermal electrons

Figure 3.4: HESSI angular overage as a fun tion of photon energy.

3.2 HESSI Instrument Subsystems

19

from 34 m to 2.75 mm in steps of the square root of 3 (see the Table of grid parameters
at URL http://hesperia.gsf .nasa.gov/hessi/referen e/Charts/Spe s.html). This gives angularresolutions that are spa ed logarithmi ally from 2.3 ar se to >3 ar min, allowing sour es
to be imaged over a wide range of angular s ales (see Figure 3.4). Di use sour es larger than
3 ar min are not imaged but full spe tros opi information is still obtained. Multiple smallersour es are imaged regardless of their separation.
The grid material and thi knesses have been hosen to provide modulation to energies ashigh as possible onsistent with maintaining a 1Æ FOV. Only two maximum thi kness grids
were hosen, providing -ray imaging while minimizing the loss in sensitivity for -ray line
spe tros opy. Thus, with the grid parameters sele ted, imaging is possible with 2.3 ar seresolution to 40 keV, 7 ar se to 400 keV and 36 ar se to >10 MeV (see Figure 3.4).
Even allowing for the grid absorption, the e e tive photopeak area for high resolution -ray
spe tros opy is still 80 m2 at 1 MeV (Figure 3.7).
A metering stru ture is used to maintain the relative twist of even the nest grid pair towithin one ar minute. This employs exural elements to mount the grids to the grid trays andthe teles ope tube to the spa e raft in order to maintain the alignment following thermal and vibrational stresses. The approa h was used su essfully in the Hard X-ray Imaging Spe trometer(HXIS) on SMM, where the alignment requirements were signi antly more stringent.
open solid angle (gt 30 keV)TABLE of spatial and energy resolution??

3.2.2 Germanium Dete tors
The dete tors used on HESSI are the largest urrently available hyperpure (n-type) germanium
dete tors (HPGe) { 7.1 m in diameter and 8.5 m long. They over the entire X-ray to ray energy range from 3 keV to 20 MeV with the highest spe tral resolution of any presently
available dete tor (<2 keV below 1 MeV to 5 keV at 20 MeV). The keV spe tral resolution of
germanium dete tors is ne essary to resolve all of the solar -ray lines (with the ex eption of
the neutron deuterium line, whi h has an expe ted FWHM of only 0.1 keV). It is also required
to resolve the detailed features of the X-ray ontinuum spe trum su h as the steep super-hot
thermal omponent and the sharp breaks in the nonthermal omponent at higher energies.
The Germanium dete tors (GeDs - see gure 3.5) have three ele tri ally-independent segments. The front 1 m thi k segment an measure hard X-rays up to 200 keV with low ba kground. The rear 7 m thi k segment provides undistorted high-resolution -ray line measurements, even in the presen e of very intense hard X-ray uxes in large ares (these are absorbed
in the front segment). Finally, there is a <0.5 m \guard-ring" at the bottom whi h in reases
the resistan e of the dete tors to ontamination.
The Germanium dete tors are ooled to their operating temperature of 75 K by a single
ele tro-me hani al ryo ooler. Studies at UCB have shown that radiation damage is signifi antly lowered by maintaining the GeDs at <75 K instead of the normal >85 K, andavoiding warmups above 85 K and/or high voltage y ling. Additionally, a low-altitude equatorial orbit (38Æ ) was hosen to minimize damage to the GeDs from the harged parti les inthe Earth's radiation belts. With these measures, the umulative radiation dose to theGermanium dete tors during a three-year mission lifetime is low enough to avoid noti eableradiation damage to the dete tors. Thus, neither shielding nor dete tor annealing should beneeded, although annealing (whi h an reverse the a e ts of radiation damage e e ts) is possible if ne essary. The absen e of a thi k, and ne essarily heavy, shield in this orbit, made thelightsat approa h feasible.

3 HESSI INSTRUMENT GUIDE

20

Figure 3.5: Outline of the Germanium Dete tor used on HESSI

3.2.3 Attenuators
Solar Flares have a wide dynami range of intensity over the energy range below 100 keV,
the energy range of interest for the HESSI front segments. Within that energy band there
are both thermal and non-thermal phenomena of great s ienti interest. The design goal for
HESSI is to preserve as mu h of the non-thermal photons as possible for spe tros opy andimage synthesis, while sampling the thermal phenomena with appropriate sampling te hniques.
Sin e the thermal phenomena range from the barely dete table to those that would saturate
the dete tors, spe ial te hniques must be employed. Typi al thermal parameters may range
from a temperature of 6 MK with an emission measure of 1048 m 3 to 25 MK and 1050 m 3 ,yielding a di eren e of over 8 orders of magnitude at 10 keV in the photon ux.
The original HESSI design in the SMEX proposal sa ri ed mu h of the low energy sensitivity intrinsi to the germanium dete tors in order to preserve full range spe tros opi measurements (possible only for photon ux rates below 50,000/se ) during all but the most intenseares. The design was modi ed during Phase A with the addition of a pair of moving attenuators over ea h dete tor to expand the dynami range for imaging spe tros opy to in lude far
fainter sour es.
The system onsists of two independent frames arrying one attenuator per dete tor. Ea hframe has two positions, open and losed. In the open positions, X-rays pass freely throughopenings in the frame whi h are slightly larger than the area of the front segment of ea hdete tor. In the losed position, ea h of the nine dete tors is overed by an attenuating disk
onsisting of a thin layer of aluminum. The 9 disks on ea h frame are not identi al, and are also
di erent for the two frames. The two attenuators are designated as the \Thin" and \Thi k"shutters, with the Thin shutter having less material than the thi k shutter. There are fourstates of the full attenuator system: both Thin and Thi k out, Thin in and Thi k out, Thin out
and Thi k in, and both Thin and Thi k in - these are designated with the shorthand notation
00, 10, 01, and 11. Details of the attenuator design and the material along the Xray path with both shutters in the open position are given on the HESSI web pages at URL

3.2 HESSI Instrument Subsystems

21

Figure 3.6: Attenuator transmission as a fun tion of energy
http://hesperia.gsf .nasa.gov/ri hard/shutter.html.
The attenuators have the greatest e e t on low energy transmission, below 100 keV { seeFigure 3.6, and the dashed lines for the front segments in Figure 3.7.
As a are in reases in intensity, the attenuation is automati ally modi ed by swit hing the
shutters in and out keeping the ux on the dete tors below the maximum spe tros opy ux,
while maximizing the sensitivity to the extent possible. The plate holding the attenuators is
moved using Shape Memory Alloy (SMA) a tuators. The swit h only takes about a se ond, but
sin e the SMA springs need to be ooled below their transition tempertaure (100 degC) before
the opposite swit h an be made, a waiting period of 5 minutes is imposed between opening
and losing the attenuators. Note: the time of the swit h is re orded in telemetry, but thephotons re orded during this interval are urrently not automati ally reje ted by the softwareand the user should be areful when a umulating a ross su h hanges.

3.2.4 Solar Aspe t Sensor (SAS)The primary purpose of the Solar Aspe t Sensor (SAS) is to provide high-resolution, highbandwidth aspe t information that is used for image re onstru tion.
The SAS is similar to the aspe t system on the HEIDI instrument, whi h demonstrated
0.5 ar se performan e at balloon altitudes. It onsists of three identi al lens- lter assemblies
mounted at 120deg intervals on the forward grid tray. These form full-Sun images on three
2048 element  (13 m)2 linear CCD arrays mounted on the rear grid tray. Simultaneous

22

3 HESSI INSTRUMENT GUIDE

exposures of three hords of the fo used solar images are made at sele ted periods ranging from
8 to 60 ms. A digital threshold algorithm is used to sele t N pixels that span ea h solar limb
for in lusion in the telemetry. Determination of these six limb rossings lo ations provide by
the three subsystems de ne pi th/roll o sets of the Sun in the rotating frame to better than
0.4 arse .
When the spa e raft spin axis is pointing to within 0.2Æ of Sun enter, simple algorithms
using the limb pixel numbers also provide real-time error signals with 10 ar se pre ision to
the spa e raft Attitude Control System (ACS). Use of the SAS in this way for both imaging
and pointing avoids problems of oalignment. The SAS is also used as a solar a quisition sensor
with an e e tive radial eld of view of 46 ar min by the dete tion of a single limb in any oneof the three diode arrays.
Although the aspe t solution itself is independent of twist, the internal onsisten y of thethree independent solutions (possible with the built-in redundan y) provides a ontinuous,
highly sensitive measure of the relative twist of the upper and lower grid trays during ight.
The SAS aspe t requirement of 1.5 ar se orresponds to a sensitivity to relative twist of 0.4
ar min. During prelaun h oalignment tests, the twist determined by the SAS was alibrated
against that provided by the Twist Monitoring System (TMS; modelled on a system developed
for the SMM-HXIS instrument). This method would identify any twisting at the tray level { iftwists o urred within a grid pair, this would be more diÆ ult to determine.

3.2.5 Roll Angle Sensor (RAS)
For image re onstru tion, a knowledge of relative roll is required at all times to 3 ar min (3sigma). Sin e all sour es of torque on the spa e raft are weak, the required information an
be obtained with a star s anner that samples the roll orientation at least on e per rotation.
Interpolation between measurements allows the roll orientation to be determined at intermediatetimes with the required a ura y.
The Roll Angle Sensor (RAS) onsists of a 2048-element linear photodiode array and ele troni s (nearly identi al to those of the SAS) behind an f/1.0, 50-mm lens. A sunshade limits
the FOV so that a 30Æ band is swept out a ross the sky orthogonal to the spin axis. As the
spa e raft rotates, ea h dete ted star generates a brief spike in the output of one or two pixels,
the timing de nes the roll orientation. For +2 magnitude stars, the dete tion signal-to-noise is15:1. Allowing for Earth o ultation and the re overy time from anti ipated Earthshine saturation, at least one (and typi ally seven) su h star(s) will be dete ted ea h rotation throughout
the mission. Measurements of only one star, averaged over a minute, allow the roll angle to be
determined to 2.7 ar min (3 sigma).

3.2.6 Data Formats
Event Pa kets: HESSI data, whether a are is a tive ot not, largely onsists of event pa kets.

These in lude the energy and arrival time of ea h event, together with information about the
spa e raft aspe t and rotation.
If the ondoard memory starts to ll up, a de imation algorithm throws out all but one ofevery N events in the front segments below a ertain energy E, with N from 2{16. E and N are
fun tions of both the remaining memory and the position of the attenuators. De imation in
the rear segments an be ommanded as a routine way of keeping ba kground (mostly photons
from the Earth's atmosphere or osmi di use ba kground) from lling up the memory. For
example, a high level of de imination an be ommanded for times when HESSI is in the Earth's

3.3 HESSI Instrument Performan e

23

shadow.
The time tag of an event uses the last 10 bits of the time ounter. If the ount rate is low
enough that the last 10 bits roll over between events, dummy \time stamp" events are inserted
so that the time on ea h events is not ambiguous.

Monitor Rates: These are a separate kind of data pa kets { ea h pa ket onsists of 10 1-

se ond a umulations of a set ounters for ea h of the 18 segments (9 dete tors, ea h witha front and rear segment). The ounters in lude ountrates for the lower and upper level
dis imintators (LLD and ULD) and the fra tion of \live" results from the livetime strobe. The
information is used to he k the health of the dete tors and to make livetime orre tions to
the data. Note: Livetime a e ts only the intensity and not the shape of the spe trum (see 3.3.2).

Fast Rates: Fast rates are produ ed when the ount rates are very high. The data are ountrates in four broad energy bands (all in the hard X-ray range). The pulses are sampled fromthe fast ele troni s hain. The rates for the three dete tors (1,2,3) with the nest grids (andtherefore fastest imaging modulations) are sampled at 16 kHz; the next three (4,5,6) at 4 kHz;and the three oarsest grids (7,8,9) at 1 kHz. Events are not shut o when fast rate data turnon. However, at these very high ounts rates, the event data will naturally taper o due to very
high deadtime.
3.3

HESSI Instrument Performan e

3.3.1 Sensitivity
The HESSI total e e tive areas for the front and rear dete tor segments are shown in Figure
3.7. The e e ts of the attenuators on the response of the front dete tor segments an be seen(dashed lines), these segments are sensitive to lower energy photons.
Figure of front and rear spe tra from a typi al HESSI dete tor ????
HESSI arries small onboard radioa tive sour es (137 Cs) whi h produ es a line at 662 keV,
far from any line expe ted to o ur in ares or in HESSI's variable ba kground. The known
intensity of the line makes it possible to monitor hanges in the eÆ en y of the dete tors, su h
as might be aused by radiation damage. The ount rate from the sour e is so small that it
an only be dete ted in spe tra a umulated over many hours and therefore does not a e t theobservations.
Be ause the dete tors are ooled, the e e ts of radiation damage are expe ted to be minimized.

3.3.2 Issues that a e t performan e
Pile-up Reje tion: Closely spa ed events an ause pulse pile-up in the analogue ele troni s.

This is be ause the ele troni s have not nished pro essing one event before the next one arrives.
To redu e this problem, events less than 6 s apart are reje ted outright, while for those withseperations between 6 and 9 s, the se ond event is reje ted { this is a e ted by the tail of the
rst event.
Pile-up results mainly from lower energy photons. In the front segments, events below about
6 keV (whi h don't trigger the lower level dis riminators in the fast ele troni s) an pile up
unresti tedly on other events. Pile-up adds a wedge of ounts at the lower end of the spe trum
and an also add peaks to the spe trum at higher energies that are \ghosts" of lower energy
peaks.

3 HESSI INSTRUMENT GUIDE

24
103

Total Effective Area (cm2)

102

101

100

Ge Front Segments

10-1

Ge Rear Segments
10-2

10-3
1

10

100 keV
1
Photon Energy

10100 MeVFigure 3.7: HESSI total e e tive areas for the front and rear dete tor segments.
Pile-up orre tion is an iterative pro ess based on the shape of the raw spe trum and knowledge of the ountrate and measured livetime. It orre ts the shape of the spe trum, and removes
the extra higher energy peaks. For the majority of ares, pile-up orre tion will be unne essary,and for most of the remainder, rst-order orre tions will suÆ e.

Livetime: Livetime is independent of energy, and the livetime orre tion hanges only theoverall magnitude of the spe trum (i.e. photometry) rather than the spe tral shape. However,
livetime orre tions must be applied onsistently to all dete tor segments to avoid indire tly
modifying the shape of the spe trum.
The livetime of a dete tor is simply the probability that one additional ount, put in at a
random time, would in fa t be re orded. The livetime signal for ea h segment is derived fromags from di erent parts of the ele troni s indi ating they are busy. The signal is sampled at
1 MHz and the livetime reported in the monitor rates is the fra tion of the sample strobes thatare lear. Note: Deadtime is (1.-livetime), the per entage time that a ir uit is \tied up" byother events.
Be ause the pile-up ir uit an sometimes veto both events, there is a loss of livetime thatannot be measured just by sampling the pile-up reje tion logi line. The livetime orre tion
routines therefore do not use the \raw" livetime measurements as reported, but a orre ted
livetime, whi h is somewhat lower.
The maximum event throughput is 25-30 thousand ount per segment per se ond and is
rea hed at about 50% livetime.

Ba kground Subtra tion: HESSI is an unshielded, high-ba kground instrument and it istherefore ne essary to subra t ba kground even in the ase of bright ares.
The simplest way to estimate the ba kground is to average spe tra from just before and justafter a are. This is the default method provided in the software, and the user is asked (see

3.4 Image re onstru tion

25

Se tion ??) to pi k intervals that should be used as ba kground from a light urve ( ountrate
vs. time).
In ases when the are lasts the better part of an orbit, or o urs just after sunrise, or justbefore sunset, this te hnique is diÆ ult to apply. Using ba kground from the orbits before andafter the are, at the same orbital phase as the are, is one way to solve this problem. Otherte hniques are being developed.

Image events: When a gamma-ray event moves through the dete tor, the hole and ele tron

louds liberated in one segment an indu e harge in the other segment. The event is orre tly
dete ted in the rst segment, but an image event an also be dete ted in the other (empty)
segment. The di eren e is that the image harges reverse sign in the empty segment as thelouds approa h the ele trodes of the segments in whi h they are a tually moving. The result
is that the image signals in the empty segements are bipolar in shape and integrate to zero.
The HESSI ele troni s only sample for a nite time (a few mi ro se onds), and a small
amount at the very end of ea h pulse is not ounted. The sampled pulse is normally negative(???), and sin e it is the negative part of the bipolar pulse that omes last in the empty segment,
the result is a very small, positive residual from the bipolar signal. In other words, a 1 MeV
event dete ted in one segment will reate a simultaneous (but false) event of a few keV in theother segment.
Thus, you may also hear image events referred to as \bipolar pulses" or \false oin iden es".

Other on iden es: There are two other ways events an be oin ident between two segments(or two dete tors): there ould be real energy deposits in both segments from the same initial
photon or osmi ray (\true on iden es"), or there an be separate photons that arrive atnearly the same time by han e (\a idental oin iden es").
The simplest way to do spe tros opy is to throw away all oin ident events { de ned as events
within N binary mi rose onds of ea h other (where the best value of N is 4 or 5). However,in order to dete t faint gamma-ray line, you may want to in lude front/rear oin iden es, andon iden es between adja ent segments (the software has a ag for this??). Also, when theountrates are very high, you may not want to reje t oin iden es at all sin e a large fra tionof the events will be a idental oin iden es.
At moderate ountrates, true oin iden es an be separted from image events by pre ise
relative timing of the two events.
3.4

Image re onstru tion

As the HESSI spa e raft rotates, the signal produ ed by a sour e is modulated by the rotating
grids { the way it is modulated hanges depending on the sour e position, intensity and size.
In Figure 3.8, the right hand panel shows single sour es of di erent sizes and intensities at
di erent lo ations relative to the spin axis (marked with a \+"), and the left hand panel shows
the resulting modulation patterns.
The top line shows the modulation resulting from a sour e whose diameter is mu h lessthan half the separation distan e (P) from the axis of rotation. If the intensity of the sour eis halved (2nd line), the amplitude of the modulation is redu ed. If the the distan e of thesour e (of original intensity) from the axis of rotation is hanged (3rd line), the frequen y of
the modulation pattern is modi ed. When the sour e is kept at the same distan e as the top
line, but rotated around the axis of rotation (4th line), the phase of the pattern shifts (although
its form is the same at the rst line). When a sour e is pla ed at the same lo ation as the top

26

3 HESSI INSTRUMENT GUIDE

Figure 3.8: Demonstration of how the modulation hanges with sour e position, intensity andsize.
line, but its size in reased (lines 5 and 6), the amplitude of modulation de reases about its
mean value until (when the sour e size ex eeds half the seperation distan e) is it barely visible.
Of ourse, the reality of a tual observations mean that sour es have omplex shapes of nonuniform intensity and the resulant modulation pattern is mu h more omplex (e.g. the bottomline). The image re onstru tion te hniques try to re reate the original sour e stru ture using
the information ontained in the modulation patterns observed in the di erent ollimators.
Also see the explanation about image re onstru tion on the Web pages under URL
http://hesperia.gsf .nasa.gov/hessi/hessi show image.htm.

3.4.1 Sele ting the sub- ollimators and energy range
It is important to keep in mind some of the limitations of the imaging te hnique when re ontru ting HESSI images and analyzing the data.
At rst, it might seem that the best results are obtained using all the sub ollimators sin ethis results in the largest number of total ounts available for onstru ting the image; however,this is generally not the ase: Sub ollimators with a FWHM resolution mu h greater than the sour e extent will only
indi ate integrated ux, but will not a tually in rease the signal-to-noise (S/N) of the re-3.4 Image re onstru tion

27

onstru ted image. The FWHM resolution in of the ollimators (in ar se onds) are given
below:
Collimator 1
2
3
4
5
6
7
8
9
FWHM
2."3 3."9 6."9 11."9 20."7 35."9 62."1 107" 186"

 Sub ollimators with a FWHM resolution greater than or equal to the imaging eld ofview are not useful for the ba kproje tion algorithm, or any other algorithm whi h relies onba kproje tion as the basis for the imaging. Thus, when imaging relatively small regions of
a are, ollimators 8 and 9 provide no information on the morphology of the sour es.
 Sub ollimators with a FWHM resolution less than the diameter of the smallest feature in
the image also add no information on the sour e morphology but do ontribute noise to themap.
 Sub ollimators with a resolution less than 2 times the pixel size will introdu e alias features
into the map, and should never be used.
 Sub ollimators that are transparent at the energy band sele ted for the map only indi ate
integrated ux and add no information on sour e morphology.
These onsiderations suggest that weighting among sele ted sub ollimators ould be very
useful, and an ertainly help ba kproje tion and CLEAN image quality. Two possible weightings hemes are available.
Aside from the hoi e of sub ollimators whi h will optimize a given image, there are spe tral
onsiderations for the imaging to be aware of as well. In some ways, it is diÆ ult to separate
the spe tral aspe ts of the imaging from the imaging itself. This leads to a few fa ts the user
should be aware of. The rst onsideration then is imaging spe tros opy. Many users will
want to make images of elds whi h ontain more than one sour e and then wish to obtain a
spe trum of ea h sour e. The urrent way to approa h this task is to make images in a numberof di erent energy ranges and then add up the ounts in ea h sour e to onstru t the spe trumof the sour es.
A related onsideration then is just how broad an energy range to use when onstru tingan image. Sin e you get more ounts and hen e higher signal-to-noise ratio, it might be assumed
that the broader the energy range, the better - however, this is not ne essarily the best hoi e.
The HESSI imaging software uses all the photons dete ted between the spe i ed energy range
to determine the modulated light urve from ea h dete tor for the spe i ed time interval.
The software then assumes a mean energy for this energy range for omputing the expe ted
response of the instrument whi h is applied to the data to ompute the image. However, sin e
the modulation patterns themselves are a fun tion of energy, there is no unique instrument
response that works for all energies. Therefore, it is not a good idea to onstru t images withan arbitrarily wide energy range.
One way to atta k this problem is to make several images with smaller energy ranges andompare them. As long as the stru ture of these various images remains the same, it is probably
OK to then make a nal image en ompassing the whole energy range.

3.4.2 Testing the Image Quality
An important aspe t of image analysis using HESSI data is to be able to de ide whi h featuresin the image an be trusted. Some fa tors to onsider when trying to judge whi h features of28

3 HESSI INSTRUMENT GUIDE

an image to trust are the following:
A. A reasonable fra tion of the total ounts should be in the feature of interest (a smaller
fra tion is more sus eptible to systemati alibration errors). Counting statisti s - statisti alsignal to noise in the peak of a ba kproje tion map is proportional to SQRT(total ounts);
B. Image morphology - too omplex an image stru ture may not be real (simpler is better);C. Unusual sour e variability: if the sour e is hanging rapidly in relative intensity a rossthe image, this ould produ e onfusing results;
D. Instrumental e e ts ould also a e t the image quality. In addition to any hardware
or software anomalies, these might in lude:dead-time - does something in the image hange as the dead time in reases?;fast-rate mode - does a new feature appear when the instrument goes into fast-rate mode?;attenuator hanges - does a new feature oin ide with a hange in the attenuator position?;sour e lo ations - do any lo ations hange relative to pointing axis or imaging eld of view;
suspi ious symmetries - are there any ar s, et . relative to the pointing axis or strong
image sour es.
Any unusual hara teristi s of the aspe t solution should also be onsidered.

29

A Useful HESSI SoftwareB

Web Version of the HAG

The HESSI Analysis Guide (HAG) may be viewed as a Web do ument on the URL's:

 http://umbra.nas om.nasa.gov/bentley/guides/hag/ http://yda .mssl.u l.a .uk/guides/hag/
The HAG is available as a PostS ript do ument; it is in two formats through the Web:for A4 paper, and for US Letter paper. Choose the one appropriate for your printer - it isdesigned to be printed double-sided. If you want a printed opy of the HAG, the PostS ript
version is re ommended sin e the s reen representation of spe ial hara ters in the Hypertext
version does not produ e good printed output.
The HAG was prepared at the YDAC (lo ated at the Mullard Spa e S ien e Laboratory,
University College London) by Bob Bentley. The Hypertext version was translated from
LaTeX using TtH, and further formatted using IDL.
This version of the HAG was prepared on: May 31, 2001

Indexhsi obs summary, 3
light urve, 6
spe trum, 10

An illary Data, 12Attenuators, 20
Ba kground, 22, 24

Pile-up reje tion, 23
Pointing, 16
RAS, 22
SAS, 21
PostS ript version, 29

Catalog
An illary Data, 3
HESSI Data Catalog, 1
HESSI Flare List, 1, 3
HESSI Observing Summary, 1, 3
Coin ident Events, 7, 25
a idental, 25
false, 25
true, 25
Cumulative radiation dose, 19

Re onstru tion te hniques
Ba k Proje tion plus Clean, 7
Forward Fitting, 9
MEM-Sato, 8
MEM-Vis, 8
Pixon, 9

Data Formats, 22
Event Pa kets, 22
Fast Rate, 22

Sensitivity, 23SHOW SYNOP, 12
Shutters, 20
Spa e raft Pointing, 16
SSW- omplient index, 6E e tive area, 23
Environment variables, 3Germanium Dete tors (GeDs), 19
alibration, 23
radiation damage, 19
Grids, 17parameters, 17
relative twist, 22

Useful software, 29

HESSI Flare List, 4
HESSI GUI, 13
HESSI Observing Summary, 3
HESSI Synopti Data Ar hive, 12
HESSI IMAGE, 7
HESSI SIM FLARE, 14
HSI IMAGE, 7
Image events, 25
Image Re onstru tion, 7, 25
Imaging System, 17
livetime, 23
Map Obje ts, 6, 12
Obje ts
hsi are list, 430